The present invention relates to a manufacturing method of a thin-film magnetic head with a magnetoresistive (MR) sensor especially using spin valve effect, used for HDD (Hard Disk Drive) units.
Recently, thin-film magnetic heads with MR read sensors based on spin valve effect of giant MR (GMR) characteristics are proposed (U.S. Pat. Nos. 5,206,590 and 5,422,571) in order to satisfy the requirement for ever increasing data storage densities in today""s magnetic storage systems like hard disk drive units. The spin valve effect thin-film structure includes first and second thin-film layers of a ferromagnetic material separated by a thin-film layer of non-magnetic material, and an adjacent layer of anti-ferromagnetic material is formed in physical contact with the second ferromagnetic layer to provide exchange bias magnetic field by exchange coupling at the interface of the layers. The magnetization direction in the second ferromagnetic layer is constrained or maintained by the exchange coupling, hereinafter the second layer is called xe2x80x9cpinned layerxe2x80x9d. On the other hand the magnetization direction of the first ferromagnetic layer is free to rotate in response to an externally applied magnetic field, hereinafter the first layer is called xe2x80x9cfree layersxe2x80x9d. The direction of the magnetization in the free layer changes between parallel and anti-parallel against the direction of the magnetization in the pinned layer, and hence the magneto-resistance greatly changes and giant magneto-resistance characteristics are obtained.
The output characteristic of the spin valve MR read sensor depends upon the angular difference of magnetization between the free and pinned ferromagnetic layers. The direction of the magnetization of the free layer is free to rotate in accordance with an external magnetic field. That of the pinned layer is fixed to a specific direction (called as xe2x80x9cpinned directionsxe2x80x9d) by the exchange coupling between this layer and adjacently formed anti-ferromagnetic layer.
In this kind of spin valve effect MR read sensor structure, the direction of the magnetization of the pinned layer may change in some cases by various reasons. If the direction of the magnetization changes, the angular difference between the pinned and free layers changes too and therefore the output characteristic also changes. Consequently stabilizing the direction of the magnetization in the pinned layer is very important.
In order to stabilize the direction of the magnetization by the strong exchange coupling between the pinned and anti-ferromagnetic layers, a process of temperature-annealing (pin anneal process) is implemented under an external magnetic field with a specific direction. The pin annealing is done as follows, first the temperature is elevated up to the Neel point under the magnetic field strength of 500 Oe to 3 k Oe, and held for about 30 minutes to 5 hours, and then cooled down to room temperature. By this pin anneal process, the exchange coupling is regulated at the interface of the pinned and anti-ferromagnetic layers toward the direction of the externally applied magnetic field.
However, the magnetoresistance characteristics may be changed under actual high temperature operation of a hard disk drive unit, even if the pin anneal processing is properly implemented. This degradation is caused by the high temperature stress during operation of the hard disk drive unit and by the magnetic field by a hard magnet layer used for giving a bias magnetic field to the free layer.
The detail of this degradation is as follows. The pinned direction of the magnetization in the pinned layer is different from that of the magnetic field (HHM) by the hard magnet. And hence the direction of the magnetization of pinned layer which is contacted with the anti-ferromagnetic layer is slightly rotated toward the direction of HHM (hereinafter the direction of the magnetization of the pinned layer is expressed as xcex8p). In the anti-ferromagnetic material layer, the Neel point temperature differs from location to location inside the layer from macroscopic point of view, and it is distributed in a certain range of temperature. Even if the temperature is less than the xe2x80x9cbulkxe2x80x9d Neel point (average Neel point), there could be small area whose micro Neel point temperature is low and where the exchange coupling with the pinned layer disappears. When such spin valve effect MR read sensor is operated at a high temperature T, which is less than the blocking temperature at which the exchange couplings of all microscopic areas disappear, and then cooled down to usual room temperature, some microscopic area whose Neel temperatures are less than T is effectively annealed again and the direction of the magnetization is rotated to xcex8p. The total amount of the xcex8p rotated area by the temperature cycle determines the magnetic structure of the anti-ferromagnetic layer and the new direction of the magnetization of the pinned layer.
As stated in the above paragraph, usage of such spin valve MR read sensor at high temperature may cause a change of the pinned direction of the magnetization in the pinned layer, and the electrical output characteristics of the sensor are degraded in signal levels, and waveform symmetry.
It is therefore an object of the present invention to resolve the aforementioned problems, and to provide a manufacturing method of a thin-film magnetic head apparatus, whereby pinned direction can be kept in stable at high temperature.
The present invention provides a manufacturing method of a thin-film magnetic head with a spin valve effect MR read sensor including a temperature-annealing step (pin anneal process) of firmly fixing the direction of the pinned magnetization in the spin valve effect MR sensor. The temperature-annealing step is executed by a plurality of times.
Going into detail, the present invention provides a method of manufacturing a thin-film magnetic head with a spin valve effect MR sensor, including a step of forming the spin valve effect MR sensor having first and second layers of a ferromagnetic material separated by a layer of non-magnetic material, and an adjacent layer of anti-ferromagnetic material formed in physical contact with the second ferromagnetic layer, and a step of temperature-annealing (pin anneal process) the spin valve effect MR sensor under a specific magnetic field with a defined direction to enhance the exchange coupling at the interface of the second ferromagnetic layer and the adjacent layer. The temperature-annealing steps are executed during wafer fabrication of the MR sensor by a plurality of times.
Twice or more executed pin anneal processes during wafer fabrication (from formation of spin valve effect thin-film structure on a wafer to just before dicing of the wafer into bars) give stronger exchange coupling and hence a spin valve effect MR sensor with more stable direction of the magnetization of the pinned layer under high temperature atmosphere i s realized. By stabilizing the direction of the magnetization of the pinned layer, the degradations of signal level and waveform symmetry of output waveforms under high temperature atmosphere c an be greatly reduced.
The temperature-annealing can be done in an independent dedicated process from the wafer fabrication, or in a part of another heat treatment process of the wafer fabrication, or in both of an independent specialized process from the wafer fabrication and a part of another heat treatment process of the wafer fabrication.
At the independent dedicated process for temperature-annealing under a magnetic field, the heat treatment temperature is elevated to a specified point (Neel temperature of the anti-ferromagnetic material of about 150 to 300xc2x0 C.) and sustained for a specified duration time, and then it is cooled down to room temperature (about 20 to 30xc2x0 C.).
An anisotropic magnetization applying process onto the first ferromagnetic layer is also preferred for the previously mentioned another heat treatment process. The anisotropic magnetization applying process may include a process for elevating the heat treatment temperature to a specific point and for sustaining it for a specified duration time under a specific magnetic field. The temperature-annealing step may be a step of cooling the heat treatment temperature down from the sustained point to room temperature (about 20 to 30xc2x0 C.) under a magnetic field toward the pinned direction.
The another heat treatment process may be a resist curing process. The resist curing process may include a process for elevating the heat treatment temperature to a specific point and for sustaining it for a specified duration time under a specific magnetic field. The temperature-annealing step may be a step of cooling the heat treatment temperature down from the sustained point to room temperature (about 20 to 30xc2x0 C.) under a magnetic field toward the pinned direction.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.